Signal Conditioning of Thermistor

Introduction quest conditioning of thermistor Signal conditioning means manipulating an analogue argue in such a way that it meets the requirements of the next stage for hike processing. Operational amplifiers(op-amps) argon commonly employed to carry out the intricacy of the signal in the signal conditioning stage. The signal conditioning equipment whitethorn be required to do li skinny processes comparable amplification, attenuation, integration, specialism, addition and subtraction.They ar also required to do non-linear processes like modulation, demodulation, sampling, filtering, clip descentg and clamping squaring, linealizing or multiplification by a nonher routine etc. the signal conditioning or info acquisition equipment in many a situation be an inflammation and amplification brass for passive transducer. It may be an amplification system for active transducer. In both the applications, the transducer produce is brought up to a ample direct to fetch it usefu l for conversion, processing, indicating and recording.Excitation is needed for passive transducers because these transducers do not gene tell their own potentiality or afoot(predicate). wherefore passive transducers like strain gauges, potentiometers, confrontation thermometers, inductive and capacitive transducers required excitation from out-of-door sources. The active transducers like techno generators, thermocouples, inductive pick ups and piezo-electric crystals. The thermistor constitute cardinal subsection or much than cardinal arm of a wheatstone bridge circuit which is excited by an isolated DC source. The bridge tin be labyrinthine sensed by a potentiometer and behind also be calibrated for un equilibrize conditions.Thermistor is a ducking of the term Thermal impedance. It is essenti whollyy a semiconductor, which behaves as a resistor with a lavishly negative temperature coefficient of enemy. That is, as the temperature of the thermistor increases, its imm oneness decreases. The temperature co-efficient is expressed in ohms per whole mixture in head Celsius ( C). thermistors with superior temperature co-efficient of vindication atomic number 18 more sensitive to temperature dislodge and be therefore well suited to temperature measurement and control.CONTENTS OBJECTIVE EXPLAINATION 1. WHEATSTONE BRIDGE 2. THERMISTOR 3. OPERATIONAL AMPLIFIER 4. INSTRUMENTATION AMPLIFIER METHODOLOGY chain 1. Wheatstone bridge Whetstone bridge is the most accurate method available for metre electrical shields and is popular for laboratory use. The circuit draw of typical Wheatstone bridge is condition in figure Rx is the noncitizen resistance to be measured R, R2 and R are resistors of known resistance and the resistance of R2 is adjustable.If the ratio of the two resistances in the known leg (R2 / R) is equal to the ratio of the two in the unknown leg (Rx / R3), then(pre nominative)ce the potential difference between the two midpoint s (B and D) will be zipper and no new will f commencement through the galvanometer Vg. R2 is vary until this condition is reached. The direction of the current indicates whether R2 is besides high or too low. Detecting zero current loafer be through with(p) to highly high accuracy (see galvanometer). Therefore, if R, R2 and R3 are known to high precision, then Rx back end be measured to high precision.Very small lurchs in Rx disrupt the balance and are readily detected. At the point of balance, the ratio of R2 / R = Rx / R3 Therefore, Alternatively, if R, R2, and R3 are known, but R2 is not adjustable, the emf difference across or current flow through the meter can be employ to cypher the value of Rx, using Kirchoff s Circuit laws (also known as Kirchhoff s rules). This setup is frequently used in strain gauge and resistance thermometer measurements, as it is usually faster to read a potential difference level off a meter than to adjust a resistance to zero the emf.In p ractical Wheatstone bridge, at least one of the resistance is made adjustable, to permit balancing. When the bridge is balanced, the unknown resistance (normally committed at Rx) may be determined from the setting of the adjustable resistor, which is called a standard resistor because it is a precision device having very small tolerance. Rx= (R2/R1). R3 APPLICATION OF WHEATSTONE BRIDGE A Wheatstone bridge may be used to measure the DC resistance of various types of wire, either for the purpose of quality control of the wire itself or of some assembly in which it is used.For example, the resistance of motor winding, transformers, solenoids, relay coils and resistance of thermistor, RTD also can be measured. PRINCIPLE Resistance of thermistor tacks with change in temperature. Resistance of thermistor decreases with increase in temperature while resistance of thermistor increases with decrease in temperature. It is a thermally sensitive resistor. design- Rti = Rt2Exp p (1 / T1 1 / T2) Where,RT= resistance of thermistor at absolute temp. Tlk RT2= resistance of thermistor at absolute temp. T2k P= a constant depending upon the material of the thermistor (Typically between 3500k to 4500k)CONSTRUCTION- Thermistors are composed of sintered mixture of metallic oxides such as Manganese, Nickel, Cobalt, Copper, press out and Uranium. They are available in a variety of sizes and shapes. They may be in the form of beads, roads and discs. WORKING- A thermistor change in electrical resistance due to a corresponding temperature change is evident whether the thermistors dust temperature is changes as result of conduction or radiotherapy from the surrounding environment or due to self heating brought slightly by queen dissipation within the device. THEORY- .Thermistor is a concentration of the term Thermal Resistor. It is essentially a semiconductor which behaves as a resistor with a high negative temperature coefficient of resistance. That is, as the temperature of th e thermistor increases, its resistance decreases. The temperature co-efficient is expressed in ohms per unit change in degree celcius ( C). thermistors with high temperature co-efficient of resistance are more sensitive to temperature change and are therefore well suited to temperature measurement and control. Thermistors are available in a round-eyed variety of shapes and sizes.However, thermistor beads sealed in the tips of glass rods are most commonly used because they are comparatively easy to mount TYPES OF THERMISTOR- 1PTC 2NTC NTC (Negative Temperature Coefficient) It implies that the resistance of thermistor decreases with increases in its temperature. These thermistors can detect changes in temperature, which could not be observed with RTDs or thermocouple circuits. NTC type thermistors mostly used in industry. Resistance of thermistor is used in industry is 2. 2 k Q. for temperature 30C. Its cost near about Rs. 10-15 SPECIFICATION- *Range(? ) 50-300 *Accuracy 1 *Resolut ion 0. 01 ? C * photographic plate Non-Linear *Thermister Constant 4000? K OPERATIONAL AMPLIFIRES One type of amplifier, which finds its way into about all points of measurement and instrumentation system, is the operational amplifier. The word operational indicates that the amplifier can perform mathematical operations like inversion addition, subtraction, multiplication, division, integration and differentiation etc. Properties of exalted operational amplifier are 1. It should have an infinite stimulus impedance. 2. It should have zero output impedance. 3.It should have an infinite assume (gain of the secernate of 105 to 109) 4. It should have flat response over a wide frequency range. The operational amplifier consists of a very high gain amplifier with a negative feedback. The gain of operational amplifier with negative feedback is determined by feedback components and not by the internal amplifier circuitry. APPLICATIONS OF OPERATIONAL AMPLIFIER Some of the important appl ications of an op-amp are 1. Amplifiers 2. prompt filters 3. Arithmetic circuits 4. Log and antilog amplifiers 5. Voltage comparators 6. Waveform generators 7.Precision rectifiers 8. Multipliers 9. Timers 10. Multivibrators 11. Regulated power supplies Operational amplifier characteristics 1. foreplay equilibrate potential difference The arousal offset electric potential is defined as the voltage that must be applied to the gossip terminals to drive the output to zero. This is about 2mV for 741 amplifier. It should be understand thet the offset voltage changes with temperature. 2. Input offset current just as a voltage offset may be required across the scuttlebutt to make the output voltage zero, so a net current may be required between the excitants to zero the output voltage.This current is called comment offset current. This is equal to the difference between the two stimulation currents. 3. Input bias current It is defined as the mean of the two input currents require d to make the output voltage zero. 4. Slew rate it is the highest rate at which the output can change, it is expressed in terms of v/jiS. 5. Unity gain frequency in many cases, specifications include the frequency response including unity gain frequency. This is the frequency at which the open coil gain of the amplifier becomes unity. The low frequency gain is about 20,000 and falls to unity at about 1MHz. he amplifier is said to have a 1 MHz gain bandwidth produt. 6. Common path rejection ratio (CMMR) it is defined as the ratio of differential gain to common mode gain. CMRR is infinite for ideal op-amp. Thus the output voltage corresponding to the common mode fray is zero. IDEAL CHARACTERISTICS OF OP-AMP 1. Gain is maximum. 2. Input impedence should be infinite. 3. rig impedence should be zero. 4. CMRR should be infinite. 5. Bandwidth should be infinite. TYPICAL VALUES OF OP- AMP 1. Input offset voltage 6mV. 2. Input offset current 200nAmp 3. Input bias current 7nAmp 4.Differ ential input resistance/impedence 2m CI 5. Input impedence for 741 (FET op-amp) 1000 G Q 6. sidetrack impedence 75 Q. 7. Gain 2 lakhs 8. Output voltage swing 26Vpp 9. grant current 2. 8 mAmp APPLICATIONS- 1. It is used in lab and medical purpose. 2. PTC type thermistors are used to protect the motor from overheating. 3. It gives very accurate reading at high temperature. 4. For measurement of level pressure, flow of the liquid, composition of gases, thermal conduction and vaccum measurement. UA741 General-purpose single operational amplifier UA741 General-purpose single operational amplifierFeatures * too large input voltage range * No latch-up * High gain * bypass protection * No frequency compensation required * Same pin configuration as the UA709 Applications * Summing amplifiers * Voltage followers * Integrators * Active filters * Function generators exposition The UA741 is a high performance monolithic operational amplifier constructed on a single silicon chip. It is inte nded for a wide range of analog applications. N DIP8 (plastic package) D SO-8 (plasticmicropackage) Pin connections (top view) 2 Inverting input 3 Non-inverting input 4 VCC- 5 Offset null 2 6 Output 7 -Vcc+ N. C. The high gain and wide range of operating voltages interpret superior performances in integrator, summing amplifier and general feedback applications. The internal compensation network (6 dB/octave) ensures perceptual constancy in closed- loop circuits. 1/11 www. st. com Schematic diagram 1 Schematic diagram UA741 Figure 1. Schematic diagram infrangible maximum ratings and operating conditions 2 Absolute maximum ratings and operating conditions give in 1. Absolute maximum ratings Symbol Parameter Value Unit Vcc Supply voltage 22 V Vid Differential input voltage 30 V Vi Input voltage 15 V Output short-circuit duration Infinite Rthja Thermal resistance conjunction to ambient SO-8 DIP8 125 85 C/W Rthjc Thermal resistance junction to case SO-8 DIP8 40 41 C/W ESD HBM human personify illustration(1) DIP package SO package 500 400 V MM machine model(2) 100 V CDM charged device model(3) 1. 5 kV Tstg Storage temperature range -65 to +150 C 1. Human body model a 100 pF capacitor is charged to the specified voltage, then discharged through a 1. 5kfl resistor between two pins of the device. This is do for all couples of connected pin combinations while the other pins are floating. . forge model a 200 pF capacitor is charged to the specified voltage, then discharged directly between two pins of the device with no external series resistor (internal resistor 5 n). This is done for all couples of connected pin combinations while the other pins are floating. UA741 3. Charged device model all pins and the package are charged together to the specified voltage and then discharged directly to the ground through only one pin. This is done for all pins. 4. 4. INSTRUMENTATION AMPLIFIER The low level signal output of electrical transducers often need to be mplified before further processing. This is done by the use of instrumentation amplifier. The important features of instrumentation amplifier are as follows. 1. Selectable gain with high gain accuracy and gain linearity. 2. Differential input capability with high gain common mode rejection. 3. High stability of gain with low temperature co-efficient. 4. low DC offset and purge errors referred to input. 5. low output impedance. The input amplifiers A and A2 act as input buffers with unity gain for common mode signals ecm and with a gain of (1+2R2/Ri) for differential signals.A high input impedance is ensured by the non-inverting configuration in which they operate. The common mode (cm) rejection is achieved by the following stage which is connected as a differential amplifier. The optimum common mode rejection can be obtained by adjusting R6 or R7 ensuring that Ei Ei R4 R6 The amplifier A3 can also be made to have some nominal gain for the whole amplifier by an appropriate selecti on or R4, R5, R6 and R7. The drift errors of the second stage add to the product of the drift errors of the premier amplifier and first stage gain.Hence, it is necessary that the gain in the first stage be enough to prevent the overall drift performance from being significantly affected by the drift in the second stage. The drift difficulty of instrumentation amplifier can be improved if amplifiers Ai and A2 have offset voltages, which tends to track the temperature. The gain of an instrumentation amplifier can be varied by changing R alone. A high gain accuracy can be obtained by using precision metal film resistors for all the resistance. Figure shows a simplified differential instrumentation amplifier using a transducer bridge.A resistive transducer, thermistor, whose resistance changes as a function of some animal(prenominal) quantity such as temperature is connected in one arm of the bridge and is denoted by (Rr A R), where RT is the resistance of the thermistor and delta R is the change in the Generally, resistors Ra, Rb and Rc are selected so that they are equal in value to the transducer resistance RT at some reference condition. The bridge is balanced initially at a desired reference condition. However, as the temperature changes, the resistance of the thermistor also changes causing the bridge to unbalance (Va 4- Vb).The output voltage of the bridge can be expressed as a function of the change in the resistance of the thermistor. Let the change in resistance of the thermistor be delta R. since Rb and Rc are fixed resistors, the voltage Vb is constant. However, voltage Va varies as a function of the change in thermistor resistance. Therefore The negative sign in this equation indicates that Va